Battery protective circuit

ABSTRACT

A battery protective circuit which can ensure the safety and reliability of a rechargeable secondary battery is provided. Personal digital assistants include a main circuit ( 30 ) and a battery block ( 60 ). The battery block ( 60 ) includes a battery ( 20 ) and a current-amount control circuit ( 50 ). The battery ( 20 ) is charged via an AC adapter. The current-amount control circuit ( 50 ) includes a current and temperature detecting circuit (for example a PTC element) operative to reduce a current amount when an amount of current flowing in the battery ( 20 ) approaches a boundary value of a charge-guaranteed region in which the battery ( 20 ) is rechargeable.

TECHNICAL FIELD

[0001] The present invention relates to a battery protective circuit,and more particularly to a configuration for preventing a value ofcurrent flowing in a battery from exceeding a guaranteed current when ashort or overcharge occurs.

BACKGROUND ART

[0002] Recently, personal digital assistants such as a portabletelephone, a notebook-sized personal computer and a video camera arewidely used. These personal digital assistants use a battery forsupplying power. A rechargeable secondary battery is used for such abattery.

[0003] When current flowing in a battery increases for some reason (ashort of an electric circuit or charge at an overvoltage and the like),the battery may generate excessive heat and possibly become degraded ordamaged.

[0004] Therefore, these tools are conventionally equipped with a batteryprotective circuit for protecting the battery. An example of the batteryprotective circuit includes a PTC (Positive Temperature Coefficient)element and a thermal protector. The PTC element or the thermalprotector serves as a current and temperature detecting circuit,operating in such a manner that electric resistance thereof increases asa larger current flows in the element and temperature becomes higher,and electric resistance thereof increases rapidly to suppress currentwhen a certain temperature is reached. Further, a thermistor has itsresistance value changed as an ambient temperature rises.

[0005] The conventional battery protective circuit, however, has aproblem as described below. Referring to FIG. 12, the problem of theconventional battery protective circuit will be described.

[0006] With respect to FIG. 12, the ordinate and the abscissarespectively represent voltage and current. A represents acharge-guaranteed region in which a battery is rechargeable, and BZrepresents a protection region in which a current and temperaturedetecting circuit such as a PTC element or a thermal protector isfunctional.

[0007] The charge-guaranteed region A represents a relation betweencurrent flowing in the battery and voltage across terminals of thebattery. The protection region BZ represents a relation between currentflowing in the PTC element or the like (and the battery) and voltageacross terminals of the PTC element or the like.

[0008] The charge-guaranteed region A is a region in which the batterycan protect itself, and the protection region BZ is a region in whichthe current and temperature detecting circuit is functional.

[0009] When the value of current flowing in the battery enters theprotection region BZ for some reason, the internal resistance of the PTCelement or the like increases. As a result, an amount of current flowingin the circuit decreases.

[0010] The conventional battery protective circuit has performed itsprotecting function when heavy current flows, regardless of thecharge-guaranteed region A, as shown in FIG. 12.

[0011] Therefore, for the current value between the charge-guaranteedregion A and the protection region BZ, any safety and reliability of thebattery is not assured. Thus, unfortunately, for some types ofbatteries, the battery cannot protect itself, and in addition, thesafety and reliability of a device operated by the battery cannot beassured.

[0012] Then, the present invention is made to solve the above mentionedproblem, and its object is to provide a battery protective circuit whichcan ensure the safety and reliability of a rechargeable battery and adevice operated by the battery.

DISCLOSURE OF THE INVENTION

[0013] According to an aspect, the present invention provides a batteryprotective circuit for a rechargeable battery, including acurrent-amount control circuit including a current and temperaturedetecting circuit provided near the battery, operative to detect a valueof current flowing in the battery and an ambient temperature, and todecrease the current value when the current value and the ambienttemperature reach a value of a protection region, wherein the minimumcurrent value in the protection region is less than the maximum currentvalue in the charge-guaranteed region in which the battery isrechargeable, and the maximum current value in the protection region isgreater than the maximum current value in the charge-guaranteed region.

[0014] According to another aspect, the present invention provides abattery protective circuit for a rechargeable battery, including: acurrent-amount control circuit including a current and temperaturedetecting circuit provided near the battery, operative to detect a valueof current flowing in the battery and an ambient temperature, and todecrease the current value when the current value and the ambienttemperature reach a value of a first protection region; and aninterconnection layer supplying current to be flown in the battery,including a meltable portion to be melted and cut off when the value ofcurrent flowing in the battery reaches a value of a second protectionregion. The minimum current value in the first protection region is lessthan the maximum current value in the charge-guaranteed region in whichthe battery is rechargeable, and the maximum current value in the firstprotection region is greater than the maximum current value in thecharge-guaranteed region. The minimum current value in the secondprotection region is less than the maximum current value in the firstprotection region, and the minimum current value in the secondprotection region is greater than the maximum current value in thecharge-guaranteed region. In the interconnection layer, the meltableportion has a relatively small cross sectional area, while a portionother than the meltable portion of the interconnection layer has arelatively large cross sectional area.

[0015] Preferably, at least two or more meltable portions of theinterconnection layer are arranged.

[0016] The aforementioned battery protective circuit can decrease thecurrent value before degradation and damage of the battery, even whenthe value of current flowing into the battery increases.

[0017] As a result, the battery can surely be protected, and the safetyand reliability of the battery and the device operated by the batterycan be improved.

[0018] Further, when the current amount approaches a boundary region ofan operating condition of the current and temperature detecting circuitdue to overcharge, charge in reverse direction or the like, theinterconnection layer is melted and cut off at the time when thecurrent-amount control circuit is not yet damaged, and therefore thecurrent is interrupted.

[0019] Therefore, the undesirably high temperature of the battery can beprevented and the current-amount control circuit may not be burdened. Asa result, the overall reliability and safety of the device including thebattery and the current-amount control circuit can be improved.

[0020] According to a further aspect, the present invention provides abattery protective circuit for a rechargeable battery, including: acurrent-amount control circuit including a current and temperaturedetecting circuit provided near the battery, operative to detect a valueof current flowing in the battery and an ambient temperature, and todecrease the current value when the current value and the ambienttemperature reach a value of a first protection region; and aninterconnection layer supplying current to be flown in the battery,including a meltable portion to be melted and cut off when the value ofcurrent flowing in the battery reaches a value of a second protectionregion. The minimum current value in the first protection region is lessthan the maximum current value in the charge-guaranteed region in whichthe battery is rechargeable, and the maximum current value in the firstprotection region is greater than the maximum current value in thecharge-guaranteed region. The minimum current value in the secondprotection region is less than the maximum current value in the firstprotection region, and the minimum current value in the secondprotection region is greater than the maximum current value in thecharge-guaranteed region. In the interconnection layer, the meltableportion has a relatively small cross sectional area, while a portionother than the meltable portion of the interconnection layer has arelatively large cross sectional area. In the interconnection layer, thegreater the current value in the second protection, the shorter the timefor the meltable portion be melted and cut off.

[0021] The aforementioned battery protective circuit can decrease thecurrent value before degradation and damage of the battery, even whenthe value of current flowing into the battery increases, so that thebattery can surely be protected, and in addition, the safety andreliability of the battery and the device operated by the battery can beimproved. Furthermore, when the current amount approaches a boundaryregion of an operating condition of the current and temperaturedetecting circuit due to overcharge, charge in reverse direction or thelike, the interconnection layer is melted and cut off at the time whenthe current-amount control circuit is not yet damaged, and the currentis interrupted. Therefore, the undesirably high temperature of thebattery can be prevented and the current-amount control circuit may notbe burdened. As a result, the overall reliability and safety of thedevice including the battery and the current-amount control circuit canbe improved. Further, as the current value in the second protectionregion becomes greater, the time for the meltable portion to be meltedand cut off becomes shorter. Therefore, even when heavy current flows,the meltable portion is not melted and cut off, if the time of currentflow is short enough. As a result, even when the terminal of the batterycauses a momentary short-circuit, the meltable portion does not melt, ifthe moment is short enough. Therefore, a short circuit over such a shorttime that does not affect the safety may not result in a failure of thebattery.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a diagram showing a configuration of a main portion of aportable telephone in accordance with a first embodiment.

[0023]FIG. 2 is a graph illustrating a battery protective function by acurrent-amount control circuit 50 in accordance with the firstembodiment.

[0024]FIG. 3 is a diagram showing an exemplary configuration of the mainportion of the portable telephone in accordance with the firstembodiment.

[0025]FIG. 4 is a diagram illustrating a structure of a main portion ofa portable telephone 1 in accordance with a second embodiment.

[0026]FIG. 5A is a cross sectional view illustrating a structure of aninterconnection layer 24 in accordance with the second embodiment, takenalong line A-A in FIG. 4.

[0027]FIG. 5B is a cross sectional view illustrating the structure ofinterconnection layer 24 in accordance with the second embodiment, takenalong line B-B in FIG. 4.

[0028]FIG. 5C is a cross sectional view illustrating the structure ofinterconnection layer 24 in accordance with the second embodiment, takenalong line C-C in FIG. 4.

[0029]FIG. 6 is a graph illustrating a battery protective function by acurrent-amount control circuit 50 and interconnection layer 24 inaccordance with the second embodiment.

[0030]FIG. 7 is a diagram showing an exemplary configuration of the mainportion of portable telephone 1 in accordance with the secondembodiment.

[0031]FIG. 8 is a diagram showing an exemplary configuration of the mainportion of portable telephone 1 in accordance with the secondembodiment.

[0032]FIG. 9 is a diagram showing an exemplary configuration of the mainportion of portable telephone 1 in accordance with the secondembodiment.

[0033]FIG. 10 is a graph showing a relation between the value of currentand the time at which each element functions, in a battery protectivecircuit in accordance with the second embodiment of the presentinvention.

[0034]FIG. 11 is a graph showing a specific relation between the valueof current and the time at which each element functions, in a batteryprotective circuit in accordance with the second embodiment of thepresent invention.

[0035]FIG. 12 is a graph illustrating a problem in a conventionalbattery protective circuit.

BEST MODES FOR CARRYING OUT THE INVENTION

[0036] Embodiments of the present invention will be described withreference to the figures. Note that the same or corresponding parts inthe figures are denoted with the same reference characters and thedescription thereof is not repeated.

[0037] (First Embodiment)

[0038] Referring to FIG. 1, a configuration for battery protection inaccordance with a first embodiment will be described. It is noted thatin the following description, a portable telephone is taken as anexample of a personal digital assistant for the purpose of illustration.FIG. 1 shows a configuration of a main portion of a portable telephonein accordance with the first embodiment.

[0039] Turning now to FIG. 1, the portable telephone includes a maincircuit 30 including a processing circuit and the like for sending andreceiving a signal, a battery block 60 for supplying electricity to maincircuit 30, and a control circuit 80.

[0040] Battery block 60 includes a battery 20 and a current-amountcontrol circuit 50 for protecting battery 20. When battery 20 ischarged, an AC adapter 70 is connected to an AC adapter terminal 72. ACadapter terminal 72 is electrically connected to a node P, from whichcurrent flows into battery 20.

[0041] Battery block 60 is contained inside a housing for the portabletelephone. Alternatively, it may be formed removable from the housing ofthe portable telephone.

[0042] Current-amount control circuit 50 is configured, for example,with a PTC element which is a current and temperature detecting circuit,a thermistor or the like which is a temperature sensing element, or acomposite element thereof. The current and temperature detecting circuitas used herein refers to a PTC element, a thermal protector or the like,and it has a function of continuously detecting a current value and atemperature to control the value of current. It is noted that current isinterrupted by the current and temperature detecting circuit when acertain value of current and temperature are reached. In FIG. 1,current-amount control circuit 50 is configured with PTC element 55. PTCelement 55 is connected to the side of the negative terminal of battery20.

[0043] Main circuit 30 is connected with node P. Main circuit 30 isconfigured with an electronic component including an interconnection, aresistance, a capacitor, a coil and the like, and it is operated bypower supply from battery 20. Main circuit 30 can monitor a change inresistance value (signal S) of current-amount control circuit 50. Forexample, main circuit 30 can be configured to include a circuit forcontrolling the charge such that the voltage at node P can be keptconstant by signal S.

[0044] Furthermore, main circuit 30 includes a circuit (such as a clock)operated by supply voltage received from node R, which is electricallyconnected to AC adapter terminal 72.

[0045] Control circuit 80 controls voltage and current which is suppliedfrom AC adapter 70 to battery 20 and main circuit 30. Control circuit 80includes a resistance element RE and a transistor T. Transistor T isconnected between node R and one terminal of resistance element RE, theother terminal of which is connected to node P. Transistor T turns on inresponse to a control signal received from main circuit 30. The currentand voltage for charging the battery is controlled under the control ofcontrol circuit 80.

[0046] PTC element 55 forming current-amount control circuit 50gradually increases in electric resistance as the value of currentflowing in the element increases and the ambient temperature increases.Then, when the current value and the ambient temperature exceedprescribed values, the resistance rapidly increases. Therefore, PTCelement can operate to decrease the value of current flowing in battery20 in response to the value of current flowing in battery 20 and theambient temperature.

[0047] Referring now to FIG. 2, the battery-protecting function bycurrent-amount control circuit 50 in accordance with the firstembodiment will be described. In FIG. 2, the ordinate and the abscissarespectively represent voltage and current, A represents acharge-guaranteed region in which the battery is rechargeable withoutdamage, and B represents the protection region in which the current andtemperature detecting circuit (PTC element 55) in accordance with thefirst embodiment is functional.

[0048] The charge-guaranteed region A is a region in which the batteryis rechargeable without damage, that is, the battery can protect itself.Charge-guaranteed region A ranges from 0 ampere to F0 ampere. In thefigure, F0 is about (2+α) amperes (the current value varies with batteryrating).

[0049] The protection region B of the current and temperature detectingcircuit is a region where the battery is protected. Protection region Branges from the current value F1 to the current value F2. The currentvalue F1 is included in a boundary region of charge-guaranteed region A.In the figure, F1 is about 2 amperes, and F2 is about (12+β) amperes.

[0050] As the value of current flowing in battery 20 enters protectionregion B, the current and temperature detecting circuit operates toincrease its resistance and decrease the current flowing into battery20. At this point, the current and temperature detecting circuitperforms its protecting function from the vicinity of boundary value ofcharge-guaranteed region A of battery 20. Therefore, when the value ofcurrent flowing in battery 20 gets close to the boundary value ofcharge-guaranteed region A, the value of current flowing into battery 20can be shifted to charge-guaranteed region A.

[0051] As a result, degradation and damage of battery 20 can surely beprevented, and the overall safety and reliability of the deviceincluding the battery can be assured.

[0052] Note that the configuration of current-amount control circuit 50is not limited to the one shown in FIG. 1. As an example ofcurrent-amount control circuit 50, a thermal protector may be used.Further, as shown in FIG. 3, battery 20 may have its positive terminalconnected to PTC element 55 and its negative terminal connected to atemperature sensing element 56, such as a thermistor.

[0053] (Second Embodiment)

[0054] A configuration for battery protection in accordance with asecond embodiment will be now described. According to the secondembodiment, an interconnection layer having a meltable portion to bemelted and cut off in accordance with a current value is arranged inaddition to the above mentioned current-amount control circuit 50, forbattery 20.

[0055]FIG. 4 illustrates a structure of a main portion of a portabletelephone in accordance with the second embodiment of the presentinvention. Referring to FIG. 4, a portable telephone 1 includes ahousing 2, a printed board 10 as an insulating substrate, a main circuit30, a battery 20 and an antenna 40. A large part of antenna 40 isaccommodated in housing 2, and can be extended to protrude from housing2 when portable telephone 1 is used.

[0056] Printed board 10 is fixed to housing 2. Main circuit 30 isprovided on printed board 10. Main circuit 30 is supplied withelectricity from battery 20.

[0057] Battery 20 is fixed to printed board 20. Battery 20 has a batterycore 21 as a power generation element, an exterior member 26, a positiveterminal 22 and a negative terminal 23. Positive terminal 22 andnegative terminal 23 are electrically connected to battery core 21.

[0058] An interconnection layer 24 and PTC element 55 as acurrent-amount control circuit are also arranged on printed board 10.PTC element 55 and interconnection layer 24 are connected betweenpositive terminal 22 and main circuit 30. An interconnection layer 31 isconnected between negative terminal 23 and the main circuit.

[0059] Interconnection layer 24 is formed from copper. Aninterconnection portion 24 b at both ends of interconnection layer 24 isformed to be relatively wide, and meltable portion 24 a located at thecenter portion is formed to be relatively narrow.

[0060]FIG. 5A shows a cross section seen along line A-A in FIG. 4, andFIG. 5B shows a cross section seen along line B-B in FIG. 4. FIG. 5Cshows a cross section seen along line C-C in FIG. 4.

[0061] Referring to FIG. 5A, meltable portion 24 a is formed on printedboard 10. The cross section of meltable portion 24 a is generallyrectangular. The height of meltable portion 24 a is T1, and the widththereof is W1. Referring to FIG. 5B, interconnection portion 24 b isformed on printed board 10. The cross section of interconnection portion24 b is generally rectangular. The height of interconnection portion 24b is T1 and the width thereof is W2 which is greater than W1. Referringto FIG. 5C, the length of meltable portion 24 a is L.

[0062] The cross sectional area of meltable portion 24 a is smaller thanthe cross sectional area of interconnection portion 24 b. Therefore,when interconnection layer is supplied with current, the density ofcurrent passing through interconnection portion 24 b is relativelysmall, and the density of current passing through meltable portion 24 ais greater. Accordingly, meltable portion 24 a rapidly generates heatwhen current exceeds a prescribed value. This heat generation meltsmeltable portion 24 a. Interconnection layer 24 is thereby broken.

[0063] Smaller cross sectional area (T1×W1) of meltable portion 24 aincreases the resistance of meltable portion 24 a, so that meltableportion 24 a can be melted and cut off with a small current.Alternatively, longer length L of meltable portion 24 a increases theresistance of meltable portion 24 a, so that meltable portion 24 a canbe melted and cut off with a small current. Therefore, the value ofcurrent at which meltable portion 24 a is melted and cut off can be setby adjusting the length and cross sectional area of meltable portion 24a. For example, when height T1, width W1 and length L are respectivelyset to about 35 μm, about 150 μm and 10 mm, meltable portion 24 a ismelted and cut off at about 7 amperes. When two such interconnectionlayers are disposed in parallel, meltable portion 24 a is melted and cutoff at about 14 amperes.

[0064] Referring now to FIG. 6, the battery-protecting function byinterconnection layer 24 and current-amount control circuit 50 will bedescribed. In FIG. 6, the ordinate and the abscissa respectivelyrepresent voltage and current. A represents a charge-guaranteed regionin which the battery is rechargeable without damage, and B represents aprotection region in which the current and temperature detecting circuit(for example a PTC element, a thermal protector and the like) formingcurrent-amount control circuit 50 can function. Further, C represents aprotection region of the interconnection layer.

[0065] Charge-guaranteed region A in which the battery is rechargeablewithout damage ranges from 0 ampere to F0 ampere. In the figure, F0 isabout (2+α) amperes.

[0066] Protection region B of the current and temperature detectingcircuit ranges from the current value F1 to the current value F2, andthe current value F1 is included in the boundary region ofcharge-guaranteed region A. In the figure, F1 is about 2 amperes, and F2is about (12+β) amperes.

[0067] In this case, interconnection layer 24 is formed such thatmeltable portion 24 a is melted when the value of current flowing in thebattery becomes equal to or more than F3. Here, the current value F3 isset to a value included in the boundary region of the protection regionof the current and temperature detecting circuit. In the figure, F3 isabout 12 amperes.

[0068] In other words, in the second embodiment, interconnection layer24 is formed such that the protection region by interconnection layer 24(the current region in which meltable portion 24 a is melted and theinterconnection layer is cut off) overlaps the boundary value of theprotection region of the current and temperature detecting circuit.

[0069] If the current flowing in the battery has a current value withincharge-guaranteed region A (0 ampere to F0 ampere), the battery can becharged without damage.

[0070] When the value of current flowing in the battery entersprotection region B, the current and temperature detecting circuitoperates to increase its resistance and decrease the current flowinginto the battery. At this point, current-amount control circuit 50performs its protecting function from the vicinity of the boundary valueof charge-guaranteed region A of the battery.

[0071] Further, when the value of current flowing in the battery getsclose to the boundary value of protection region B, meltable portion 24a is melted, and interconnection layer 24 is cut off, so that thecurrent flowing in current-amount control circuit 50 and battery 20 isinterrupted. Therefore, a short between PTC elements due tocarbonization of PTC element caused by the current exceeding protectionregion B can be prevented. Furthermore, the current-limiting effect byPTC element 55 has a time delay, of which effect on battery 20 can alsobe prevented.

[0072] It is noted that in the portion where two regions overlap, forexample, in the region where charge-guaranteed region A and protectionregion B of current and temperature detecting circuit overlap (thatportion in which the current value is not less than F1 and not more thanF0), charge-guaranteed region A of the battery, which is at the leftside of these regions, is designed to function with priority. Further,in the region where protection region B of the current and temperaturedetecting circuit and protection region C of the interconnection layeroverlap (that portion in which the current value is not less than F3 andnot more than F2), protection region B of the current and temperaturedetecting circuit, which is at the left side of these regions, isdesigned to function with priority.

[0073] In this manner, according to the second embodiment, when a shortor the like causes heavy current to flow in battery core 21, PTC element55 can operate to decrease the value of current flowing in battery core21. In addition, when it comes close to such an environment that is outof the condition ensuring normal operation of PTC element 55 due tocharge at an overvoltage, charge in reverse direction or the like,meltable portion 24 a is melted and cut off.

[0074] Since this can interrupt the current, the undesirably hightemperature of battery core 21 can be prevented. In addition,degradation and damage of current-amount control circuit 50 can beprevented. As a result, the overall safety and reliability of the deviceincluding the battery can be assured.

[0075] An example of such a relation between interconnection layer 24and the current-amount control circuit is as shown in FIGS. 7 to 9. Inan example shown in FIG. 7, current-amount control circuit 50 isconfigured with PTC element 55 and thermistor 56. Interconnection layer24 and PTC element 55 are connected between the positive terminal ofbattery 20 and main circuit 30, and thermistor 56 is connected with theside of the negative terminal of battery 20.

[0076] In an example shown in FIG. 8, interconnection layer 24 isarranged between main circuit 30 and the positive terminal of battery20, and PTC element and thermistor 26 configuring current-amount controlcircuit 50 is connected with the side of the negative terminal ofbattery 20.

[0077] Further, in an example shown in FIG. 9, PTC element 55 andthermistor 26 configuring current-amount control circuit 50 is connectedwith the side with lower potential. Interconnection layer 24 isconnected between PTC element 55 and a ground terminal.

[0078] FIGS. 7 to 9 show PTC element 55 and thermistor 56 configuringcurrent-amount control circuit 50, but this invention is not limitedthereto.

[0079]FIG. 10 is a graph showing the relation between the value ofcurrent and the time at which each element may function, in the batteryprotective circuit in accordance with the second embodiment of thepresent invention. In FIG. 10, the ordinate shows the time required foreach element to start an operation, and the abscissa shows the currentvalue. Curve 110 shows the relation between the time and the value ofcurrent at which the current and temperature detecting circuit (such asa PTC element, a thermal protector or the like) configuringcurrent-amount control circuit 50 may operate. Curve 120 shows therelation between the time and the value of current necessary formeltable portion 24 a of interconnection layer 24 to be melted and cutoff. Referring to FIG. 10, as shown with curve 110, as the value ofcurrent flowing in the temperature detecting circuit (such as a PTCelement, a thermal protector or the like) becomes greater, the timerequired for current-amount control circuit 50 to start an operationbecomes shorter. Similarly, as shown with curve 120, as the value ofcurrent flowing in interconnection layer 24 becomes greater, the timerequired for meltable portion 24 a to start melting and cutting offbecomes shorter. Both of curve 110 and curve 120 are convex downward,which shows that the corresponding element takes shorter time to operateas the current value becomes greater.

[0080] The protection region of the current and temperature detectingcircuit ranges from the current value F1 to the current value F2, andthe current value F1 is included in the boundary region of thecharge-guaranteed region.

[0081] In this case, for interconnection layer 24, meltable portion 24 ais melted when the value of current flowing in the battery reaches equalto or more than F3. Here, the current value F3 is set to be a valuewhich is included in the boundary region of the protection region of thecurrent and temperature detecting circuit.

[0082] Next, three interconnection layers 24 were prepared, with 0.2 mmof the width W1, 35 μm of thickness T1, and 10 mm of the length ofmeltable portion 24 a shown in FIG. 5A. These were connected inparallel, and in this interconnection layer, the time and the value ofcurrent at which the meltable portion began melting were measured in ahigh temperature atmosphere and a low temperature atmosphere. Further,the time and the value of current at which PTC began operating weremeasured in a high temperature atmosphere and a low temperatureatmosphere. The result is shown in FIG. 11. In FIG. 11, the ordinateshows the time required for each element to start an operation, and theabscissa shows the current value. Curve 201 shows the relation betweenthe time and the value of current necessary for PTC to operate in thehigh temperature atmosphere. Curve 202 shows the relation between thetime and the value of current necessary for PTC to operate in the lowtemperature atmosphere. Curve 203 shows the relation between the timeand the value of current necessary for meltable portion 24 a to bemelted and cut off in the high temperature atmosphere. Curve 204 showsthe relation between the time and the value of current necessary formeltable portion 24 a to be melted and cut off. As can be seen from FIG.11, in the interconnection layer in accordance with this invention, asthe value of current at which meltable portion 24 a is melted and cutoff becomes greater, the time required for meltable portion 24 a tostart melting and cutting off becomes shorter. In addition, it can beseen that meltable portion 24 a is melted and cut off in a shorter timein the high temperature atmosphere compared with in the low temperatureatmosphere.

[0083] For the protective circuit in accordance with the presentinvention, an external short (so called chain-short) with not more than50 m Ω of resistance value and about one second of the duration may notmelt and cut off meltable portion 24 a, and therefore the battery can bereused.

[0084] Although the first and second embodiments have been describedabove, various modifications may be made on the embodiments describedherein.

[0085] Although a portable telephone is taken as an example of apersonal digital assistant, the present invention is not limitedthereto, and may be applied to a notebook-sized personal computer, avideo tape recorder and the like.

[0086] Any of a lithium cell, a nickel-cadmium battery and a polymerbattery may be used as battery 20.

[0087] The relation between PTC element 55 and thermistor 56 andinterconnection layer 24 which are arranged for the positive terminal 22and the negative terminal 23 of the battery is not limited to the onedescribed above.

[0088] Various kinds of material can be used rather than copper and thelike, as a material of interconnection layer 24. More specifically,interconnection portion 24 b may be formed of a material with highermelting point and meltable portion 24 a may be formed of a material withlower melting point.

[0089] Further, the number of interconnection layers 24 is not limitedto one, but may be changed properly as needed. In case where a pluralityof interconnection layers 24 are provided, any one of meltable portion24 a is surely melted, and therefore the reliability of the device isfurther improved.

[0090] Still further, the shape of the meltable portion in theinterconnection layer is not limited to the linear shape as shown. Themeltable portion may be formed, for example, to extend in a serpentineshape in order to secure the length.

[0091] In accordance with the present invention, even when the value ofcurrent flowing into the battery increases, the value of current can bedecreased before degradation and damage of the battery. As a result, thebattery can surely be protected, and the reliability and safety of thebattery and the device operated by the battery can be improved.

[0092] In accordance with the present invention, the current amountapproaches the boundary region of the operating condition of the currentand temperature detecting circuit, due to overcharge, charge in reversedirection or the like, the interconnection layer is melted and cut offat the time when the current-amount control circuit is not yet damaged,and the current is interrupted.

[0093] Therefore, the undesirably high temperature of the battery can beprevented, and in addition, the current-amount control circuit may notbe burdened. As a result, the reliability and safety of the battery andthe device operated by the battery can be improved.

[0094] In addition, in accordance with the present invention, aplurality of the interconnection layers having meltable portions may beprovided, so that current can be interrupted at respective differentpositions when it enters the protection region of the interconnectionlayer. In particular, when the identical interconnection layers areprovided in parallel, the reliability can be enhanced.

[0095] Although the present invention has been described and illustratedin detail, it is clearly understood that the same is by way ofillustration and example only and is not to be taken by way oflimitation, the spirit and scope of the present invention being limitedonly by the terms of the appended claims.

INDUSTRIAL APPLICABILITY

[0096] A battery protective circuit in accordance with the presentinvention can be applied to a portable telephone, a notebook-sizedpersonal computer, a word processor, a liquid crystal television, a VTRwith camera and the like.

1. A battery protective circuit for a rechargeable battery, comprising acurrent-amount control circuit including a current and temperaturedetecting circuit provided near said battery, operative to detect avalue of current flowing in said battery and an ambient temperature andto decrease said current value when said current value and said ambienttemperature reach a value of a protection region, wherein a minimumcurrent value in said protection region is less than a maximum currentvalue in a charge-guaranteed region in which said battery isrechargeable, and a maximum current value in said protection region isgreater than the maximum current value in said charge-guaranteed region.2. A battery protective circuit for a rechargeable battery, comprising:a current-amount control circuit including a current and temperaturedetecting circuit provided near said battery, operative to detect avalue of current flowing in said battery and an ambient temperature andto decrease said current value when said current value and said ambienttemperature reach a value of a first protection region; and aninterconnection layer supplying current to be flown in said battery,including a meltable portion to be melted and cut off when a value ofcurrent flowing in said battery reaches a value of second protectionregion, wherein a minimum current value in said first protection regionis less than a maximum current value in a charge-guaranteed region inwhich said battery is rechargeable, a maximum current value in saidfirst protection region is greater than the maximum current value insaid charge-guaranteed region, a minimum current value in said secondprotection region is less than the maximum current value in said firstprotection region, the minimum current value in said second protectionregion is greater than the maximum current value in saidcharge-guaranteed region, and in said interconnection layer, saidmeltable portion has a relatively small cross sectional area, while aportion other than the meltable portion in said interconnection layerhas a relatively large cross sectional area.
 3. The battery protectivecircuit according to claim 2, wherein at least two meltable portions ofsaid interconnection layer are arranged.
 4. A battery protective circuitfor a rechargeable battery, comprising: a current-amount control circuitincluding a current and temperature detecting circuit provided near saidbattery, operative to detect a value of current flowing in said batteryand an ambient temperature and to decrease said current value when saidcurrent value and said ambient temperature reach a value of a firstprotection region; and an interconnection layer supplying current to beflown in said battery, including a meltable portion to be melted and cutoff when said value of current flowing in the battery reaches a value ofsecond protection region, wherein a minimum current value in said firstprotection region is less than a maximum current value in acharge-guaranteed region in which said battery is rechargeable, amaximum current value in said first protection region is greater thanthe maximum current value in said charge-guaranteed region, a minimumcurrent value in said second protection region is less than the maximumcurrent value in said first protection region, a minimum current valuein said second protection region is greater than maximum current valuein said charge-guaranteed region, in said interconnection layer, saidmeltable portion has a relatively small cross sectional area, while aportion other than the meltable portion of said interconnection layerhas a relatively large cross sectional area, and in said interconnectionlayer, as a current value in said second protection region becomesgreater, time for said meltable portion to be melted and cut off becomesshorter.